1. Field of the Invention
The present invention relates to a circuit for charging a secondary cell, and more specifically to a charging circuit for charging a battery provided in a housing for containing an electronic appliance, such as may be found in connection with notebook-sized personal computers.
2. Description of the Related Art
Lately, a large number of information processing devices or communications devices are being provided with batteries (secondary cell). For example, a notebook-sized personal computer is basically AC-powered when it is connected to an AC power supply, and battery-powered when the AC power supply is not provided.
A battery in a device connectable to an AC power supply is normally charged while the device is powered by the AC power supply. Practically, when the amount of power consumed by a system load of the device becomes small, residual power is used to charge the battery. For example, the power consumption of a personal computer constantly changes with time depending on whether or not a key operation is being performed, depending on whether or not a hard disk is being accessed, depending and on the display state, etc. The charging device charges the battery when the power consumption of the personal computer becomes small.
FIG. 1 is a block diagram of the information processing device provided with an existing charging circuit. In this example, it is assumed that the information processing device is a notebook-sized personal computer (hereinafter referred to simply as a notebook PC).
An AC adapter 501 is, for example, an AC/DC converter, and supplies a predetermined direct current voltage for a notebook PC. A load 511 is a circuit containing the CPU, display device, disk drive circuit, etc. of the notebook PC. A DC/DC converter 512 generates a voltage requested by the load 511 from the output of the AC adapter 501 or a battery 513, and supplies it to the load 511. A charging circuit 514 charges the battery 513 with electric power provided by the AC adapter 501. The load 511, the DC/DC converter 512, the battery 513, and charging circuit 514 are contained in a housing 510.
With the above described configuration, when the AC adapter 501 is connected to an AC power supply, a switch SW2 is basically opened when a switch SW1 is closed, and the notebook PC is driven by the power obtained through the AC adapter 501. On the other hand, when the AC adapter 501 is not connected to AC power supply, the switch SW2 is closed when the switch SW1 is opened, and the notebook PC is driven by the power obtained from the battery 513.
The electric current (or electric power) with which the battery 513 is charged has conventionally been determined based on a parameter relating to the ability of the AC adapter 501. As an example, the charging current has been controlled such that the total electric power (sum of consumed electric power and electric power charged in the battery) supplied for the notebook PC may not exceed the allowable output power of the AC adapter 501.
FIG. 2 shows the conventional charging method. In the conventional charging method, the electric power consumed by a system load is monitored. When the monitored value becomes small, the battery 513 is charged. The "electric power consumed by a system load" is represented by a sum of the electric power consumed by the load 511 and the DC/DC converter 512, and changes with time depending on the operation of the load 511. Therefore, for example, from T1 to T2, the battery 513 is charged with a large electric current while from T2 to T3, it is charged with a small electric current. Since the amount of the electric power consumed by the system load becomes large at and after time T3, the battery 513 is not charged at all.
As described above, in the conventional charging method, the charging current or charging power depends on a parameter relating to the ability of an AC adapter. A charging circuit for determining the charging current or charging power based on the parameter relating to the ability of an AC adapter is disclosed by the U.S. patent (U.S. Pat. No. 5,723,970).
Recently, the amount of heat generated by various information processing devices such as notebook PCs, etc. has considerably increased as a results of, for example, high-speed CPUs. On the other hand, a number of users request smaller notebook PCs, etc.
However, when a housing of a device is made smaller to satisfy the users' requests, the radiation of heat is necessarily lowered. In addition, specifications for heat or temperature have become strict lately. For these reasons, it has become more important these days to design a housing of a device with more careful consideration of the relationship between the shape of the housing of a device and the amount of the heat generated by the device.
Under this situation, there are some problems to be solved with the conventional charging method. That is, in the conventional charging method, the charging current or charging power has been determined based on the parameter relating to the ability of an AC adapter as described above. However, the conventional method has not been designed with the consideration of radiation ability.